The present invention relates generally to a pixel structure of an organic light-emitting diode (OLED) display device and its fabrication method, and more particularly to a pixel structure of a full-color OLED display device and its fabrication method.
Flat-panel displays have become one of the most important electronic devices in products such as notebook computers and pocket-TVs. Among the flat-panel displays, organic electroluminescent (OEL) displays have the following advantages: light emitting, high luminous efficiency, wide viewing angle, fast response speed, high reliability, full color, low driving voltage, low power consumption and simple fabrication. Undoubtedly, this product has emerged as the display of choice in the market place. The fabrication method of a conventional full-color OLED display device includes at least the following five methods: (a) using a precision photo-mask to get a pixel array of red, green, and blue (RGB) colors in a small molecule system, (b) using color filters to filter the white light from white-light OEL elements to get the three RGB colors, (c) using light changing layer to change the original blue or purple light from a blue or purple-light OEL element into light of other colors, (d) using dielectric stacking layers of different thickness to change original wide-band spectrum light to RGB colors by the reflection and interference principles of the light, and (e) stacking devices of RGB colors on the same picture element on a two-side transparent OEL element.
US Pat. No. 5,550,066 discloses a process for making a pixel structure of a thin-film-transistor (TFT) organic electroluminescent device. FIG. 1 is a diagrammatic plan view of this conventional TFT-OEL device. As shown in FIG. 1, the TFT-OEL device 100 comprises mainly two thin film transistors 101 and 102, a storage capacitor 103, and a light emitting OEL pad 104 arranged on a substrate. The TFT 101 is the logic transistor with a source bus 105 as the data line and a gate bus 106 as the gate line. The ground bus 107 is located above the gate bus 106 and below the storage capacitor 103. The source electrode of the TFT 101 is electrically connected to the source bus 105 and the gate electrode comprises a portion of the gate bus 106. The OEL pad 104 is electrically connected to the drain of the TFT 102. The drain of the TFT 101 is electrically connected to the gate electrode of the TFT 102 that in turn is electrically connected to the storage capacitor 103. The TFT-OEL devices are typically pixels that are formed into a flat panel display.
FIG. 2 is a cross-sectional view of FIG. 1 illustrating the construction of a pixel structure of a conventional TFT-OEL device. As shown in FIG. 2, a polysilicon layer is deposited over a transparent insulating substrate 201 and the polysilicon layer is patterned into a polysilicon island 208. Next, an insulating gate layer 202 is deposited over the polysilicon island 208. A layer of silicon 204 is deposited over the insulating gate layer 202 and patterned by photolithography over the polysilicon island 208 so that after ion implantation, source and drain regions are formed in the polysilicon island 208. Ion implantation is conducted by doping with N-type dopants. A gate bus 206 is applied and patterned on the insulating gate layer 202, and a second insulating layer 212 is then applied over the entire surface of the device. Contact holes are cut in the second insulating layer 212 and electrode materials are applied to form contacts with the thin-film-transistors. The electrode material attached to the source region of TFT 102 also forms the top electrode 222 of the storage capacitor 103. A source bus and a ground bus are also formed over the second insulating layer 212. In contact with the drain region of TFT 102 is the anode for the OEL material. Then, a passivation layer 224, made of insulating material, is deposited over the surface of the device. The passivation layer 224 is etched leaving a tapered edge. The OEL layer 232 is then deposited over the passivation layer 224 and the anode layer. Finally, a cathode electrode layer 234 is deposited over the surface of the device.
There are some difficulties to overcome in order for the full-color organic electroluminescent display devices to be accepted the market of flat-panel displays. For instance, it is not easy to produce display devices of high resolution, high luminous efficiency and wide viewing angle. The brightness and luminous efficiency are not good enough for the OEL device that uses organic material of small molecules. For the OEL device that uses organic material of high molecules, although having RGB colors, the overall brightness and luminous efficiency are even worse than those of small molecules OEL device. Also, the materials used in OEL devices are not compatible with photolithography. Therefore, it is necessary to develop a simple and efficient fabrication method and structure for the full-color organic light-emitting display device.
The present invention has been made to overcome the above-mentioned difficulties of manufacturing a conventional organic light-emitting display device. The primary object of the invention is to provide a pixel structure of a full-color OLED display device. The pixel structure comprises mainly a black matrix, a color changing medium (CCM), two thin film transistors, a storage capacitor, and an OLED device structure arranged on a substrate.
Another object of the invention is to provide a fabrication method of the pixel structure of the full-color OLED display device. The processing steps for the pixel structure include the black matrix process, the island process, the gate process, the interlayer process, the color changing medium process, and the OLED deposition process.
According to the invention, the pixel structure of the full-color OLED display device uses blue organic light-emitting diodes or polymer light-emitting diodes as the electroluminescent media. The low-temperature poly Si (LTPS) thin film transistors provide a current to the OLED device and serve as an active driving device. The color changing medium changes blue light into red light or green light to form a full-color OLED. Because a color changing medium is integrated on the LTPS thin film transistors, this invention can make display devices of high resolution, high luminous efficiency and wide viewing angle.
The foregoing and other objects, features, aspects and advantages of the present invention will become better understood from a careful reading of a detailed description provided herein below with appropriate reference to the accompanying drawings.